Recent studies have shown that the transfer RNA composition of differentiated cells correlates with the amino acid composition of the protein being synthesized. This suggests that the rates of synthesis and/or degradation of specific tRNAs are controlled to match the demand for amino acids in potein synthesis. An experimental approach to this question is described which utilizes Bernfield's new micromethod for purifying radioactive tRNAs. Friend virus infected mouse leukemia cells will be used for these studies. As a possible mechanism for control of specific tRNA levels, we suggest that the level of charging of a tRNA species may act to control its rate of synthesis or degradation. Evidence for this will be sought by monitoring the effects of amino acid deprivation on in vivo levels of charging for appropriate tRNA species coupled with studies on their rates of synthesis and degradation. In addition, rates of synthesis and degradation of specific tRNAs will be measured under a variety of conditions, including exponential and stationary phases of growth and induction of "differentiation" of mouse leukemia cells by dimethylsulfoxide.